As world population increases, demand for fresh water and power will also increase. Pollutants and drought result in a shortage of fresh water in many locations. Therefore, it would be desirable to provide a process utilizing desalination and distillation combined with power generation whereby demand for fresh water and power can be simultaneously satisfied in and near coastal areas.
Desalination refers to any of several commercial processes (e.g. distillation/evaporation, reverse osmosis/membrane processes, freezing, geothermal, solar humidification, methane hydrate crystallization, and high grade water recycling) that remove the salt, minerals and other solids from water in order to obtain fresh water suitable for animal consumption, irrigation, or human consumption. Dual purpose power plants have also been utilized; however, most previous processes of desalination have been stand-alone processes. Hence, the focus has been upon energy efficiency to satisfy economics. A number of factors determine the capital and operating costs for desalination: capacity and type of facility, location, feed water, labor, energy, financing and concentrate disposal. Generally the cost of removing salt from seawater will be about 3-5 times that of removing salt from brackish water.
Distillation is the process of heating a liquid until it boils, capturing and cooling the resultant hot vapors, and collecting the condensed vapors. Evaporation is the boiling of salinous water by the addition of heat followed by the condensation of the steam by heat exchange. Evaporators may be classified as boiling or flashing. Desalination stills control pressure, temperature and brine concentrations to optimize the water extraction efficiency. Distillation techniques, although dating back to antiquity, share the following difficulties: high capital cost, and the consumption of large amounts of energy.
Flash distillation is often employed in the recovery of a solvent from a solution containing a salt or other dissolved material, for example, in desalinization of seawater to produce fresh water. Flash distillation is also employed in the chemical industry and the food industry for the concentration of liquors. In such operations, the solution being treated is commonly referred to as the brine. Multistage flash distillation heats the brine to a desired temperature in its liquid state and then effects the evaporation of the heated solution in a series of stages which are maintained at progressively decreasing pressures. The condensation of the vapor created at each stage is carried out to produce the substantially pure solvent which is withdrawn. The heat which is absorbed during condensation is often employed for the preheating of the brine prior to its expansion.
It is generally known to employ parallel trains of multistage flash evaporation units, particularly in the desalination of seawater. Such installations are able to carry out desalinization in a manner which is economically competitive with other available alternative methods of desalinization.
Reverse osmosis is a technology wherein fresh water is extracted from saline water by pressure. This is accomplished by circulating saline water under high pressure (i.e., 1000-2000 psig) around a loop. One portion of the loop is adjacent to a membrane. The membrane selectively allows water to pass through it, while preventing the passage of most ions. Effectively, fresh water is squeezed from the saline water. Excellent energy efficiency can be achieved by this method. However, reverse osmosis techniques share the following difficulties: the membranes are prone to plugging and in practice the fresh water produced is not completely free of dissolved salts.
Geothermal is a technology wherein hot water or steam is collected from hydrothermal reservoirs and transferred through a heat exchanger to a closed loop desalination system, and returned to the geothermal reservoir. The hot water in the closed loop desalination system is flashed in a flash zone to form steam and the steam is used a source of heat for desalination. Geothermal techniques share the following difficulties: such systems must be located near hydrothermal reservoirs, which may not be a co-location of seawater, high capital cost, and dependence on hydrothermal reservoirs collection rates of hot water or steam which restricts the use of this technique for large-scale production.
Solar humidification is a technology that imitates a part of the natural hydrologic cycle in that the saline water is heated by the sun's solar radiation so that the production of water vapor (humidification) increases. The water vapor is then condensed on a cool surface, and the condensate collected as product water. Variations of this type of solar process still have been made in an effort to increase efficiency, but they all share the following difficulties: large solar collection area requirements, high capital cost, and dependence on optimum weather conditions for operation, which restricts the use of this technique for large-scale production.
Accordingly, various attempts to resolve the foregoing disadvantages have been proposed. Most notably, dual purpose desalination/power plants, which are currently in use, produce fresh water by using the exhaust heat from a gas turbine as a source of heat for desalination or by using excess steam from a steam generating system used in a steam expansion turbine during low electric power demands and off peak hours as a source of heat for desalination.
Power generation using steam expansion is a common process. Conventional methods for power generation include the steam cycle, cogeneration cycle, and the combined cycle.
In the steam cycle, water is heated to produce steam at high temperature and pressure. The steam is typically superheated and expanded across a turbine to produce power. The steam will frequently be heated again and expanded across a turbine a second time. The steam will then be condensed at a low temperature and the cycle is repeated. In a dual purpose desalination/power plant the power plant's condenser is replaced by the desalination plant's heat exchanger enabling such captured heat energy to reduce the energy requirements of the desalination plant. Additional energy efficiency is improved by recovery of additional waste heat from the stack exhaust.
Power generation using gas expansion is a common process. Typically, natural gas is burned and expands across a turbine; thereby, doing work. Exhaust gases are vented through an exhaust pipe.
Additional efficiencies in energy cost and capital costs are desirable for such dual purpose desalination/power plants for obtaining potable water substantially free of trace salts, minerals, and dissolved solids in order to obtain fresh water suitable for animal consumption, irrigation, or human consumption.
Therefore, it is readily apparent that there would be a recognizable benefit from a method and apparatus for desalinating water combined with power generation utilizing the efficiencies of a dual purpose co-generation facility having reduced capital cost and reduced consumption of energy, and wherein such method and apparatus desalinates coastal or seawater.